Bukhari

And Allah the Exalted said: "But if you are prevented, then (offer) whatever sacrificial animal is available, and do not shave your heads (i.e., do not exit the state of Ihram) until the sacrificial animal reaches its place (i.e., until it is slaughtered in Mecca)." And Ata bin Abi Rabah, may Allah have mercy on him, said that whatever prevents (a person), the same ruling applies to it.

Narrated Nafi`: When `Abdullah bin `Umar set out for Mecca intending to perform Umra, at the time of afflictions, he said, "If I should be prevented from reaching the Ka`ba, then I would do the same as Allah's Apostle did, so I assume the lhram for Umra as Allah's Apostle assumed the Ihram for Umra in the year of Hudaibiya."

Narrated Nafi`: That Ubaidullah bin `Abdullah and Salim bin `Abdullah informed him that they told Ibn `Umar when Ibn Az-Zubair was attacked by the army, saying "There is no harm for you if you did not perform Hajj this year. We are afraid that you may be prevented from reaching the Ka`ba." Ibn `Umar said "We set out with Allah's Apostle and the non-believers of Quraish prevented us from reaching the Ka`ba, and so the Prophet slaughtered his Hadi and got his head shaved." Ibn `Umar added, "I make you witnesses that I have made `Umra obligatory for me. And, Allah willing, I will go and then if the way to Ka`ba is clear, I will perform the Tawaf, but if I am prevented from going to the Ka`ba then I will do the same as the Prophet did while I was in his company." Ibn `Umar then assumed Ihram for Umra from Dhul-Hulaifa and proceeded for a while and said, "The conditions of `Umra and Hajj are similar and I make you witnesses that I have made `Umra and Hajj obligatory for myself." So, he did not finish the Ihram till the day of Nahr (slaughtering) came, and he slaughtered his Hadi. He used to say, "I will not finish the Ihram till I perform the Tawaf, one Tawaf on the day of entering Mecca (i.e. of Safa and Marwa for both `Umra and Hajj).

Narrated Nafi`: Some of the sons of `Abdullah told him (i.e. `Abdullah) if he had stayed (and not performed Hajj that year).

Narrated Ibn `Abbas: Allah's Apostle was prevented from performing (`Umra) Therefore, he shaved his head and had sexual relations with his wives and slaughtered his Hadi and performed Umra in the following year.